THE ANATOMY OF STEMS. 2G1 



The old layer loses its vitality, and is pushed out- 

 wards by the new ; the accumulation thus formed 

 constituting what botanical writers have called the cu- 

 tical layers. 



The vitality of the stem of dicotyledonous plants is 

 more conspicuous in the liber than in any other part. 

 If the bark be wounded, or a portion of it be remov- 

 ed, layers gradually extend themselves from the liber 

 on each side of the wound until it is closed up ; but 

 as this is not effected in one year when the wound 

 is extensive, and as the new layers are thrown out by 

 the liber only, which is annually renewed, the cica- 

 trice, if the healed portion can be so named, always 

 resembles a hollow cone, the base of which is the ex- 

 terior of the trunk. The union of a graft, or of a bud 

 taken from one tree and implanted on another, suc- 

 ceeds only when the liber of the bud, or the graft, and 

 that of the stock, is placed in immediate contact ; the 

 union in these instances closely resembling that which 

 occurs when two raw surfaces of a living animal body, 

 or of two distinct animals, are retained for some time 

 in contact. Grew, Malpighi, Du Hamel, and others, 

 supposed that the liber annually changes, by harden- 

 ing, into the alburnum or young wood, an opinion al- 

 so maintained by Mirbel and some of the ablest phy- 

 tologists, but which is founded upon mistaken princi- 

 ples. It is through the liber, however, that the mat- 

 ter in which the new wood is formed, which annually 

 augments the diameter of the trunk and branches, is 

 secreted ; and hence the importance of this portion of 

 the bark. 



Such is the structure of the bark of the stems of 

 woody dicotyledons ; and that of the roots does not 

 materially differ from it ; any difference depending, 

 perhaps, altogether on the medium in which these two 

 parts are situated. In the bark the secreted juices of 



